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fix: use bufio.Scanner for stdio transport to avoid panic when stdio mcp server outputs a long line #464

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Merged
merged 1 commit into from
Aug 24, 2025
+7 −6
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fix: use bufio.Scanner for stdio transport to avoid panic when stdio mcp server outputs a long line #464

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13 changes: 7 additions & 6 deletions client/transport/stdio.go
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Original file line number Diff line number Diff line change
Expand Up @@ -27,7 +27,7 @@ type Stdio struct {
cmd *exec.Cmd
cmdFunc CommandFunc
stdin io.WriteCloser
stdout *bufio.Reader
stdout *bufio.Scanner
stderr io.ReadCloser
responses map[string]chan *JSONRPCResponse
mu sync.RWMutex
Expand Down Expand Up @@ -72,7 +72,7 @@ func WithCommandLogger(logger util.Logger) StdioOption {
func NewIO(input io.Reader, output io.WriteCloser, logging io.ReadCloser) *Stdio {
return &Stdio{
stdin: output,
stdout: bufio.NewReader(input),
stdout: bufio.NewScanner(input),
stderr: logging,

responses: make(map[string]chan *JSONRPCResponse),
Expand Down Expand Up @@ -180,7 +180,7 @@ func (c *Stdio) spawnCommand(ctx context.Context) error {
c.cmd = cmd
c.stdin = stdin
c.stderr = stderr
c.stdout = bufio.NewReader(stdout)
c.stdout = bufio.NewScanner(stdout)

if err := cmd.Start(); err != nil {
return fmt.Errorf("failed to start command: %w", err)
Expand Down Expand Up @@ -247,14 +247,15 @@ func (c *Stdio) readResponses() {
case <-c.done:
return
default:
line, err := c.stdout.ReadString('\n')
if err != nil {
if err != io.EOF && !errors.Is(err, context.Canceled) {
if !c.stdout.Scan() {
err := c.stdout.Err()
if err != nil && !errors.Is(err, context.Canceled) {
c.logger.Errorf("Error reading from stdout: %v", err)
}
return
}

line := c.stdout.Text()
// First try to parse as a generic message to check for ID field
var baseMessage struct {
JSONRPC string `json:"jsonrpc"`
Expand Down